It is known that, in auto glazing for example, the black-colored edge is a general requirement for direct bonding of the glass panes onto auto bodies:
1. to protect the organic adhesive against UV radiation; and
2. to guarantee an aesthetically optimal finish.
Black pastes are glass-like fusion products which are mixed with black pigments and applied in paste form to auto glass panes by the screen printing process and subsequently baked, the baking temperature being in the vicinity of the softening point of the glass substrate in order to combine stoving of the black edge with shaping of the pane.
The quality of black glass paints for heated rear windows has to satisfy particularly stringent requirements because the black edge is printed onto the float side of the glass. After curing, the busbar is generally applied locally by screen printing in the form of a silver paste to the as yet unbaked black edge layer with the object of baking both layers together, the glass panes being simultaneously pressed into their intended shape and subsequently tempered. In addition, the black edge should not leave any residues behind on the pressing tools during the shaping process.
For a layer thickness of approx. 15 .mu.m, the edge of the rear window should have a strong neutral black color, should be absolutely opaque with a smooth and compact surface and should ensure good adhesion to the glass coupled with good scratch resistance and chemical stability. The silver busbar, which is approximately 13 .mu.m thick, must not be visible under glass, but should have good soldering properties on the screen printing side to guarantee the busbar a separation strength of at least 15 kg.
Black glass pastes for auto windshields, rear windows and side windows have been used for years. They generally consist of 45 to 65% by weight of a lead-containing borosilicate glass which is often pre-colored with metal oxides in the melt and has a softening point below 600.degree. C. The opacity of the glass layer or the strength of color is guaranteed by the addition of 12 to 35% by weight metal oxides or inorganic pigments during grinding of the glass frit. The pigments are fused on the glass surface during the heat treatment of the glass, i.e. at 570.degree. to 720.degree. C. Media which are liquid at room temperature or at elevated temperature are required for the application of the glass pastes to the glass by screen printing, being available in a wide choice according to the technique applied. The media in question are water-friendly or oil media, thermoplastics or polymers. Polymers cure under the effect of ultraviolet light. Metallic zinc and tin oxide are often used as additives for the glass pastes used for rear windows to prevent the black band film from adhering to the surface of the pressing tools during shaping of the glass (US 4,684,388).
Several reactions take place in the approx. 15 .mu.m thick black band film during the heat treatment of the rear window.
The organic additives of the media evaporate or burn at temperatures of up to 450.degree. C. The glass finally reaches its softening point of around 700.degree. C. (depending on the composition of the glass) which is necessary on the one hand for the shaping process and, on the other hand, for the glass matrix of the black band to bind the pigment to the glass surface. In this phase, silver ions of the busbar can migrate through the black band and react with the tin coating of the float glass side with incorporation of the coloring oxides added. As a result, the silver busbar can become distinctly visible under glass at the black edge or the opacity in this region can be reduced. The black thus varies in color.
Accordingly, the problem addressed by the present invention was to provide a black paste which would prevent the silver from migrating during the baking process and which, at the same time, would improve the opacity of the black edge and intensify the black color.
This problem has been solved by the glass-like black paste according to the invention.